1. Field of Invention
The present invention relates to a power flow management method. More particularly, the present invention relates to a power flow management method and a controller using a zero-sequence voltage.
2. Description of Related Art
A power system is vulnerable under natural disasters such as a lighting struck and a salt damage, or other accidents such as being bumped by an object (e.g. car accidents) and dropping of wires. These situations may break transmission and distribution equipments or lines. Momentary power failures caused by the malfunctions or voltage sags caused when a fault current flows into adjacent lines can not completely be avoid for a power system. Except for the voltage sag, some common problems which will cause poor power quality such as interruption, fluctuation, flicker, swell, and harmonics can be alleviated by a static synchronous compensator (STATCOM).
The STATCOM can monitor voltages and timely inject reactive power for compensation. For example, when detecting a grid voltage is lower than a predetermined value, the STATCOM operates as a capacitor and injects reactive power into a grid to raise the grid voltage. Minimizing the affect of the reactive power to make sure the maximum power transmission efficiency is a key of a stable grid. The reactive power not only makes a local voltage lower than a rated voltage, but also increases loads of a transmission line and a transformer, and this a transmission ability of active power is limited. Therefore, by decreasing the current of the reactive power on a transmission line, a transmission capacity can be increased and simultaneously a power loss is reduced. The STATCOM is required to comply with a strict requirement of dynamic reactive power compensation and a grid code to stabilize the voltages. From an aspect of using the electricity, a momentary power failure (about one minute) or a voltage sage (about two seconds) does not have a significant impact on regular electricity using, but they may cause a lot of damages to industrial users. From an aspect of providing the electricity, keeping frequency and voltages of electricity in a safe range would be an important indicator for the operation quality of the power system.